The demand for internal double-bond olefins has recently increased. For example, 2-butene-rich feeds have been found to be useful in the production of alkylate prepared by alkylation of isoparaffins with light olefins. The desirability of using butene-2 as compared to butene-1 as feedstock to an alkylation zone to produce high octane gasoline blending stocks is disclosed in U.S. Pat. No. 2,804,490. U.S. Pat. No. 3,800,003 presents a process in which a feed stream comprising butene isomers is passed into an isomerization zone to increase the quantity of butene-2 available for passage into a downstream alkylation zone. U.S. Pat. No. 4,918,255 discloses an alkylation process using a heterogeneous isoparaffin/olefin alkylation catalyst, e.g. BF.sub.3 /Al.sub.2 O.sub.3, wherein the olefin feed is isomerized to reduce alpha olefin content using as isomerization catalyst alumina, silica, zirconia, chromium oxide, boron oxide, thoria, magnesia, aluminum sulfate or combinations thereof, as well as boron halide-modified metal oxide.
Double bond isomerization of olefins such as butene in the presence of catalysts of the pentasil type such as ZSM-5 and ZSM-11 at temperatures of 100.degree. to 500.degree. C. is disclosed in European Patent Application 0 129 899 to Hoelderich.
European Patent Application 0 247 802 to Barri et al. discloses restructuring olefins using tectometallosilicates of the Theta-1 type (ZSM-22) as well as ZSM-23 at relatively high reaction temperatures of 200.degree. to 550.degree. C. Table 4 thereof shows 1-butene to 2-butene selectivity (mol/mol) of Theta-1 catalyst in the conversion of 1-butene of 92.1% at 234.degree. C. at 100 MPa pressure using an 11.5.+-.2.8% vol/vol 1-butene in nitrogen feed.
U.S. Pat. No. 4,749,819 to Hamilton, Jr. exemplifies double bond isomerization of an alpha olefin feed (preferably C.sub.12 to C.sub.18) to produce a product having interior double bond isomerization using a ferrierite catalyst. The reference further teaches at column 5, lines 15 to 19, that "[o]ther aluminosilicates may be exemplified by ZSM-12, ZSM-22, ZSM-23 and ZSM-48."
It is not unexpected that a wide variety of catalysts can be used to isomerize 1-butene at high initial activity inasmuch as the double bond shift is one of the most facile among the hydrocarbon reactions. The thermodynamics of the reaction indicate that enhanced selectivity for 2-butenes occurs at lower temperatures and that relatively great selectivities are possible with a wide variety of catalysts at such temperatures. However, those catalysts which exhibit the desirable activity and stability for the double bond shift reaction can often produce unwanted oligomer by-products.
Accordingly, it would be desirable to provide a method for isomerizing alpha-olefins (or terminal double bond olefins) to internal double bond olefins, e.g, 1-butene feeds to 2-butene rich products, over a catalyst which exhibits high 1-butene conversion and 2-butene selectivity, while minimizing oligomer by-product formation.
It is known in the art that surface acidity of zeolitic catalysts can be modified by treatment with various base reagents. U.S. Pat. No. 4,870,038 to Page et al discloses a process for producing substantially linear hydrocarbons by oligomerizing a lower olefin at elevated temperature and pressure with siliceous acidic ZSM-23 whose surface is rendered substantially inactive for acidic reactions, e.g., by contact with 2,4,6-collidine (2,4,6-trimethylpyridine, gamma-collidine). U.S. Pat. No. 5,015,361 to Anthes et al discloses a method for catalytic dewaxing which employs zeolite catalysts surface acidity deactivated with bulky amines. The reduction in surface acidity serves to reduce the amount of lower value cracked products obtained during dewaxing. U.S. Pat. No. 4,101,595 teaches the modification of zeolites by exchange and similar technology with large cations such as N.sup.+ and P.sup.+ and large branched compounds such as polyamines and the like. As disclosed in U.S. Pat. Nos. 4,520,221 and 4,568,786, zeolites which have been surface-deactivated by treatment with bulky dialkylamines are useful as catalysts for the oligomerization of lower olefins such as propylene to provide lubricating oil stocks.
Base treatments, however are often temporary in nature and require the continual provision of base in the feedstream in order to maintain the desired deactivation of the catalyst surface. Moreover, the bulky base can obstruct pores resulting in reduced intrapore activity of the zeolite and requiring higher temperatures, which are undesirable for internal double bond selectivity, and/or reduced space velocity.
Another approach to selectively deactivating the surface of zeolites involves dealumination of zeolite surfaces. Techniques for zeolite dealumination include hydrothermal treatment, mineral acid treatment with HCl, HNO.sub.3, and H.sub.2 SO.sub.4, and chemical treatment with SiCl.sub.4 or ethylenediaminetetraacetic acid (EDTA). The treatments are limited, in many cases, in the extent of dealumination by the onset of crystal degradation and loss of sorption capacity. U.S. Pat. No. 4,419,220 to LaPierre et al discloses that dealumination of zeolite Beta via treatment with HCl solutions is limited to SiO.sub.2 /Al.sub.2 O.sub.3 ratios of about 200 to 300 beyond which significant losses to zeolite crystallinity are observed.
U.S. Pat. No. 3,442,795 to Kerr et al. describes a process for preparing highly siliceous zeolite-type materials from crystalline aluminosilicates by means of a solvolysis, e.g. hydrolysis, followed by a chelation. In this process, the acid form of a zeolite is subjected to hydrolysis, to remove aluminum from the aluminosilicate. The aluminum can then be physically separated from the aluminosilicate by the use of complexing or chelating agents such as ethylenediaminetetraacetic acid or carboxylic acid, to form aluminum complexes that are readily removable from the aluminosilicate. The examples are directed to the use of EDTA to remove alumina.
EP 0 259 526 B1 discloses the use of dealumination in producing zeolite ECR-17. The preferred dealumination method involves a combination of steam treatment and acid leaching, or chemical treatments with silicon halides. The acid used is preferably a mineral acid, such as HCl, HNO.sub.3 or H.sub.2 SO.sub.4, but may also be weaker acids such as formic, acetic, oxalic, tartaric acids and the like.
U.S. Pat. No. 4,388,177 to Bowes et al. discloses the preparation of a natural ferrierite hydrocracking catalyst by treatment with oxalic acid to impart catalytic activity for converting slightly branched as well as straight chain hydrocarbons in hydrodewaxing and naphtha upgrading. Increased activity is believed to arise from removal of iron, sodium and other impurities by such treatment.
As far as is known, surface-dealuminated zeolites have heretofore not been used as double-bond isomerization catalysts.